As is known, two implantation techniques are available for insertion of femoral prostheses. On the one hand, femoral prostheses can be implanted in the appropriately prepared proximal region of the femur by the cementation technique. In this case, the prosthesis stem is positively anchored in the bone bed using bone cement. On the other hand, appropriately shaped femoral prostheses can be implanted by the cementless technique. In this case, the aim is frictional anchoring ("pressfit") of the prosthesis stem with the bone bed.
The present invention is directed at a femoral prosthesis which is intended for implantation by the cementation technique and provides considerable improvements for this in respect of surgical technique, implantation result and useful life of the prosthesis.
Despite the current high state of development in respect of optimization of prosthesis design, composition and processing characteristics of bone cement systems, surgical and cementation techniques, the implantation result is still influenced by many situational factors and, moreover, depends to a high degree on the skill of the operator. Considerable problem points in this connection are the centering and fixing of the prosthesis appropriate for the anatomy, and the choice of the cement characteristics appropriate for the situation.
When commercially available bone cements of high viscosity are used, the insertion, centring and fixing of the prosthesis in the prepared bone bed which is provided with a bed of cement can be performed more easily and reliably because the prosthesis is scarcely able to change position again until the curing process is complete. However, homogeneous and pore-free mixing of high-viscosity bone cements is difficult. Application, for example using a bone cement press or gun, requires a large force to be exerted. In addition, it is scarcely possible, because of the low flowability, to produce a completely continuous cement covering between bone bed and prosthesis surface. However, this is precisely what is absolutely necessary for firm and permanent bonding and thus a long and useful life of the prosthesis.
Low-viscosity bone cements which are likewise commercially available can be processed considerably more easily and reliably in this respect. However, the positioning and fixing of the prosthesis are difficult so that, as a rule, additional measures are necessary to stabilize the seating of the prosthesis.
Further developments in cementing with low-viscosity cement aim at initial insertion of the prosthesis into the bone bed and correct positioning therein using centring devices, and only then filling the remaining free space in the bone bed with cement. In this case, the bone cement can be applied using a cement gun and/or by suction in with the aid of vacuum. In a further embodiment of this cementation technique, the cementing takes place after the insertion and fixing of the prosthesis by transprosthetic application. Femoral prostheses suitable for this have a longitudinal borehole extending proximally to distally in the stem, through which the bone cement is injected in order then to fill the bone bed advancing distally to proximally. Reference may be made to EP 0 434 604 A1 for an example of this technique. However, this technique also has disadvantages which may affect the success of the operation and the useful life of the prosthesis. Thus, for example, it cannot be precluded that the prosthesis surface is contaminated with blood and discharge and/or irrigation fluid, or that bleeding takes place into the applied cement covering which is undergoing curing. This results in a considerable weakening of the prosthesis/cement covering/bone bed bonding, so that there may be premature loosening and breaking out of the implant.